The invention relates to a method of hot rolling a metal strip or metal sheet, particularly a steel strip or steel sheet, wherein    1.a) the slab or the strip or sheet is subjected in a first step to one or more reshaping actions in a hot rolling mill for producing a uniform, fine-grain austenite structure and    2) the slab or the strip or sheet is then subjected in a further step to cooling for producing a fine-grain structure.
The production of steel strip or steel sheet by hot rolling has been described in the prior art for a long time. Corresponding disclosures are found in, for example: the paper of P. Uranga, A. I. et al., ‘Improvement of Microstructural Homogeneity in Thermomechanical Processed Ng Steels by Thin Slab Casting’, 43rd Mechanical Working and Steel Processing Conference. Charlotte, ISS, Vol. 39, pages 511 to 529; the paper of C. Klinkenberg at al., ‘Processing of Niobium Microalloyed API Grade Steel on a Thin Slab Plant’, Materials Science Forum, Vols. 500 to 501, 2005. pages 253 to 260; and the paper of S. V. Subramanian et al., ‘Process modelling of microalloyed steel for near net shape casting’. Proceedings of the International Conference on Thermomechanical Processing: Mechanics, Microstructure, ed. by E. J Palmiere et al., The University of Sheffield, Sheffield, 2003, pages 148 to 156.
A typical application of the thermomechanical method (TM method) is the production of hot-rolled steel strip and steel sheet from microalloyed steels. Such a TM treatment usually consist of the following steps:
One or more reshapings of the austenitic initial structure are carried out in the recrystallisation temperature range in a first step 1.a for producing a uniform, fine, recrystallised austenite structure. One or more reshapings of the recrystallised, austenitic structure for producing a high-dislocation, non-recrystallised austenite structure extended in area as much as possible (so-termed pancake structure) can then be carried out in a step 1.b. This first step is also termed conditioning of the austenite.
Finally, cooling of the austenitic structure conditioned by means of the first step is carried out in a second step for producing a fine-grain structure in the finished hot strip or hot sheet with the phase conversion.
The structure of the finished hot strip or hot sheet consists of the combination of ferrite, perlite, bainite and martensite, wherein the content of these four structural components can respectively be between 0% and 100%.
In accordance with the aforesaid description of the TM method, it is also possible to dispense with the afore-mentioned step 1.b of reshaping in the non-recrystallising temperature range of the austenite. In this case, conditioning of the austenite takes place entirely in the recrystallising temperature range of the austenite.
However, the difficulty with thermomechanical hot rolling is that for producing the uniform fine recrystallised austenite grain in the recrystallising range (the above step 1.a) a largest possible reshaping has to be undertaken. A fine recrystallised structure is distinguished by the fact that not only the former, non-uniform cast structure, but also individual coarse grains or structure regions have to be completely transformed into a uniform, fine recrystallised structure with small scatter around the mean grain size. This condition frequently is not fulfilled or is fulfilled incompletely and leads to an inadequately conditioned austenite structure.
If a step 1.b in the non-recrystallising temperature range of the austenite follows on the first step 1.a of hot rolling in the recrystallising temperature range of the austenite, often only a little residual reshaping for the succeeding reshaping actions in the said step 1.b of the thermomechanical treatment remains when there is a large ratio of the thickness of the finished hot strip or hot sheet and the thickness of the slab or the intermediate strip. Even when individual stands are taken out of action this on occasion is not sufficient to transform possibly still present residues of the cast structure as well as individual coarse grains or structure regions into a uniform flat pancake structure of non-recrystallised austenite grains. An insufficiently conditioned austenite is also present in this case.
Inadequately conditioned austenite has the disadvantageous consequence in the finished hot strip or hot sheet of individual coarser grains outside the normal distribution about the mean grain size and/or of structure regions having a substructure characterised by small-angle grain boundaries. However, structure regions of that kind lead to deterioration in the mechanical properties of the strip or sheet, particularly to a reduced toughness.
in correspondence with the preceding description of the TM method, in the above-mentioned step 1.a of the method the reshaping in the recrystallising range of the austenite is of critical importance for the characteristics of the finished steel strip or sheet. The degree of reshaping required in step 1.a of the thermomechanical treatment can, in fact, be replaced in part by an elevated entry temperature. However, this possibility is limited by the maximum furnace temperature as well as by the cooling at the time of contact with the roll and by thermal radiation between the stands when several stands participate in this step.